This invention relates to a method and apparatus for detecting a predetermined characteristic of data signals and, particularly, for regenerating such signals in mobile radiotelephone systems.
Numerous schemes are known in the art for regenerating received baseband data in order to make the binary coded information contained therein available for use in equipment that need not operate in time synchronism with the received data. However, such schemes are usually characterized by various shortcomings, at least in regard to mobile radiotelephone systems, in that the circuits are either highly complex and, thus, too expensive for employment in individual mobile units; or the circuit operation is inadequate in an error rate sense for coping with the vagaries of the multipath, Rayleigh fading environment in the near microwave region of the frequency spectrum.
In a U.S. Pat. No. 4,010,323 to R. J. Peck, a timing recovery circuit, apparently for a fixed facility system, samples data at a rate higher than the bit rate and applies the samples to a shaft register. The phases between samples separated by an integral number of carrier wave half-cycles along the shift register are compared in order to derive data signal phase. The timing information thus derived is then utilized in circuits not disclosed.
A further U.S. Pat. No. 4,029,900 to E. J. Addeo illustrates one example of circuits known in the art for recovering timing from Manchester coded signals in a mobile radiotelephone environment. A large portion of the circuit operates in an analog signal mode and is, therefore, subject to soft failure in that, e.g., circuit element aging can cause filter mistuning with consequent degradation in operation. Also, it has been found that sometimes unresolved phase ambiguities can occur during certain data sequences.
In a paper entitled "Individual Implementation of an FFSK Modem," pp. 30.2.1-30.2.5, in the Conference Record of the 1978 National Telecommunications Conference, R. Matyas et al. describe a modem for use in mobile radio applications. A phase-comparing circuit used there is responsive to data signal level transition pulses and to a reference wave for determining whether the received signal is leading or lagging in phase. An up/down counter averages the lead/lag outputs of the phase comparator to determine the extent of required local clock reference correction. Multiple bit intervals appear to be necessary to acquire phase. Although the system is described as operating satisfactorily in the VHF range, it has been found that such systems with heavy dependence on information transitions are subject to errors due to noise when required to operate in the near microwave range, e.g., between 800 and 900 MHz.